Question Stopping notification sounds from making videos/music/etc. more quiet

Aug 24, 2019
Until December, I've used a Lenovo C2 phone, for which there were no more Android updates available after 6.0. On that phone, the following problem did not occur.

In December, I've bought a Redmi Note 8 Pro with Android 9 PPR1.180610.011, which is way better than my old phone in pretty much every way. I have one incredible annoyance with it though: Whenever a notification sound is played while I'm watching a video (on YouTube, in Firefox, and locally alike) or listen to music or an audio file, the audio goes down to about 30% of its normal volume, the notification sound (which is only a second long!) plays, and the audio won't go back to its normal volume for about four seconds. It recently began driving me crazy, and I couldn't find any way to turn it off in my notification or sound settings. If anyone knows how to just make the notification sounds play over other audio without changing the volume, that would be very much appreciated.